Tropical Storm Rosa threatens 12 million with flooding as southwest US braces for 'life-threatening' conditions

The storm is already reportedly responsible for at least one death

Tropical Storm Rosa drenched northwestern Mexico as it churned towards California and the southwest US, where it can be expected to cause floods and heavy rain.

The storm has already reportedly claimed one victim, as residents in Arizona and Utah filled sandbags in anticipation of the heavy rainfall forecasts and potential flooding.

The National Weather Service earlier announced flash flood watches through Wednesday for areas including southern Nevada, southeastern California, southwestern and central Utah and the western two-thirds of Arizona.

Forecasts call for heavy rainfall in the watch areas, which include Las Vegas, Phoenix and Salt Lake City, with possible flooding in slot canyons and normally dry washes and a potential for landslides and debris flows from recent wildfire burn scars.

National Weather Service meteorologists in Phoenix said central and northern Arizona stood to get hit with the heaviest amounts of precipitation.

Metropolitan Phoenix, where temperatures were above 100 degrees Fahrenheit (38 Celsius) only a few days ago, had already cooled to the 80s thanks to Rosa. Moisture associated with the storm arrived ahead of it, bringing rain Sunday night and Monday morning.

The centre of Rosa, which was a hurricane until late Sunday, was expected to hit Baja California and Sonora by early Tuesday, bringing 3 to 6 inches of rain, the US National Hurricane Centre said.

Rosa was then expected to move quickly northwestward as it weakens, bringing 2 to 4 inches of rain to central and southern Arizona and 1 to 2 inches to the rest of the desert Southwest, Central Rockies and Great Basin. Isolated areas might reportedly see even more precipitation.

The storm’s maximum sustained winds had decreased to 40mph as of Monday night, and the storm’s centre was about 75 miles north of Punta Eugenia in Mexico. It was heading northeast at 10 mph.

The Civil Defence agency for Baja California state said schools would be closed Monday in several communities, including the state capital of Mexicali, across the border from Calexico, California; San Felipe, on the northern Sea of Cortez; and south of Ensenada, on the peninsula’s Pacific coast.

Federal authorities declared a state of emergency for Ensenada and Mexicali.

Classes were also suspended in communities in Sonora, where Civil Defence officials advised people to avoid driving on the coastal highway.

The agency posted video online of floodwaters racing through an arroyo and covering a stretch of road, carrying trees and logs in the current.

Various Mexican media outlets reported that a woman was swept away by floodwaters and drowned in the city of Caborca, Sonora, on the Sea of Cortez.

Phoenix meteorologist Jaret Rogers said more showers were expected across central Arizona later in the day before a weakened Rosa arrives in the area Tuesday morning.

According to the National Weather Service, the entire Phoenix area was expected to see between 1 and 2 inches of rain. But some areas could see between 2 to 3 inches.

Several cities posted on social media that they were distributing sandbags.

Meanwhile, Tropical Storm Sergio became a hurricane in the Pacific, though it posed no immediate threat to land. Sergio had winds of 65 mph Sunday afternoon and was centered about 535 miles south-southwest of Manzanillo, Mexico. The storm was moving west at 13mph.

The Associated Press contributed to this report